The present invention relates to estimation of sound level.
Many rooms or halls have a sound system installed, so that, for example, music or announcements can be played out to the audience or guests in the hall. Similarly, a concert venue would have a PA system installed, a movie theater would have a cinema sound reproduction system, and a studio would have its audio monitoring system.
A modern studio, cinema or home theater would have some type of surround sound system, supporting at least 5.1 or 7.1 channel sound reproduction. Bars, cafés, clubs, and discotheques would typically have a “house” sound system installed, comprising numerous loudspeakers—tens or even hundreds. In larger installations, different audio sources could be played in different zones, such that the “audience” at two different locations would hear either the same audio but at different sound levels, or different audio entirely.
In all the above cases, it is desirable to know the sound pressure level (SPL) at different positions in the acoustic environment, which the sound system covers. The SPL could be monitored, for instance to ensure that a maximum SPL is not exceeded. Or the SPL at different locations or zones could be checked against the intended SPL in each zone. Monitoring the SPL may be useful for both the owner of the venue, the arranger of the event, and the audio engineer, DJ, or operator mixing or controlling the levels of the sound sources and/or of the amplifiers powering the loudspeakers.
To measure the SPL, at some position, a sound level meter is normally employed (e.g. as specified in IEC 61672). Either a stand-alone device (e.g. Brüel & Kjær's Sound Level Meter—type 2250), or a measurement-microphone with omnidirectional characteristic, connected to sound level measurement software running on a PC having an audio i/o-interface with calibrated A/D converters installed. FIG. 1 illustrates a prior art embodiment where a sound level meter is employed to measure the SPL in an acoustic environment. FIG. 2 illustrates a prior art embodiment where the SPL is measured at two different listening positions. If the SPL is steady or reproducible the SPL at the listening positions may be measured sequentially by using only one sound level meter, but if the SPL needs to be monitored or maximum SPL during some period needs to be determined, then a sound level meter is required for each listening position.
A stand-alone sound level meter, often handheld or setup on a tripod, would be well-suited for “spot checks” of the SPL. But making such a device a permanent installation would be impractical—both due to the cost of purchase and maintaining the device, and due to the inconvenience of having, for example the sound level meter on a tripod installed at the center of a café or concert venue. Maintenance would include ensuring correct operation of the device, as well as calibration of sensitivity vs absolute level, at regular intervals.
Moreover, the measurements would need to be collected or downloaded onto a central storage in order to keep records of the SPL corresponding to different days or events. Having only the measurement-microphone at the location of interest and the actual sound measurement device stored somewhere else, would be another approach. In this case, the microphone and the measurement device would need to be connected by a cable carrying the audio signal and usually “phantom power” to the microphone.
In a real acoustic environment, i.e. with furniture and reflective surfaces of different materials, the combined effect of the multiple loudspeakers would mean that the resulting SPL, as well as coloration of the sound, could vary considerably between different locations. Hence, multiple sound level meters would be required—one for each measurement location—each with its own measurement microphone, hence suffering from the same problems as a single location, only multiplied.
When measuring the SPL, several different measures may be of interest: The peak SPL would reflect the maximum instantaneous acoustical level. Furthermore, the SPL may be averaged over the most recent (say) 15 minutes time period, and reported as an Leq, “equivalent continuous sound level”, often A or C frequency weighted. Regional authorities may limit the maximum permitted Leq over one or more time periods, for public events.
The integrated sound pressure is generally known as Sound Exposure, and may further be A-weighted and calculated for a normalized 8 hour working day (ISO 1999; IEC 61252). Such measures are employed in regulations by national and international authorities to control and limit noise-induced hearing loss (NIHL), especially related to noise (including music) in the workspace. A recent example is: Directive 2003/10/EC of the European Parliament and of the Council of 6 Feb. 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise).
Some prior art methods exist for estimating and limiting e.g. the SPL that the user of a certain headset or personal media player would be exposed to. They are based on prior knowledge of technical characteristics, such as sensitivity of the amplifier and/or transducer in the specific device. Often they employ rather crude means, such as simply limiting the voltage of the output signal or limiting the amplification gain of the device. These procedures do not directly correspond to controlling or limiting the actual Sound Exposure. FIG. 3 illustrates a prior art personal sound system with sound level control, where an SPL/SE estimator uses knowledge about the audio amplifier and the headphone transducers to control the SPL based on the fact that the sound will be delivered right into the user's ears and not interact with a more open acoustic space.
U.S. Pat. No. 8,737,630 discloses a system for estimating the sound exposure to reduce risk of noise induced hearing loss when listening to music through earphones or headphones. The estimation may be based on microphone measurements in the earphone or measurements of the audio signal before or after the amplifier. U.S. Pat. No. 7,013,011 discloses a system for limiting the possible sound pressure level which a user may experience from a telephone headset. The system uses a predetermined transfer function including the amplifier, headset and ear coupling characteristic to estimate the SPL which the input audio signal will cause, and attenuates the input audio signal if the estimated SPL is above a threshold.
A problem with the prior art methods is that they are bound to the specific device, for instance, a certain MP3-player or a certain headset. Furthermore, a most severe limitation is that these prior art methods cannot be applied to loudspeakers in an acoustic environment, because they are based on the assumption that the transducer (e.g. headset or in-ear headphone) is “coupled directly” with the ear of the user/listener. This is even more limiting when considering acoustic environments with multiple loudspeakers interacting in producing varying sound levels at different positions, as opposed to the headphone environments with basically one loudspeaker per ear.
Other prior art devices exist for measuring the Sound Exposure that one person is exposed to, typically during a working day; such a device may be known as a Personal Noise Dosimeter. These devices are self-contained and portable, and must be “worn” the whole time by the user, typically on his or her shoulder. Each device would contain a microphone used to continually measure the noise or sound level, to which the user is exposed, and then accumulate and display the “dose”. If the dose would exceed a certain limit, the device could issue a warning, and the user might for example be encouraged to wear hearing protection.
Three principal limitations of such devices are: 1) the initial cost, 2) the maintenance (e.g. calibration and recharging), and 3) the discipline required for each person to always possess and wear the dosimeter.